Slow response time and poor viewing angle of the conventional TFT-LCD are two of the major limitations for its otherwise potentially unlimited wide range of applications.
FIG. 1 shows the structure of a conventional TFT-LCD. A liquid crystal (LC) layer 10 is sandwiched between a top glass substrate 11 and a bottom glass substrate 12. A thin layer of transparent electrode, indium tin oxide (ITO), is coated on each substrate for the application of an electric field to switch the liquid crystals. Normally, the electrode 13 for the top substrate 11 is a common electrode, which has a constant voltage (e.g. 0V). 0V is used herein to mean low voltage. This common electrode is continuous and extends to all pixels in the whole display and hence is called a common electrode. On the other hand the electrode 14 on the bottom substrate 12 is called the pixel electrode since a transistor assigned to each individual pixel controls it. The voltage applied to the LC 10 is varied through this electrode. The electric field profile E is also shown in FIG. 1, when the pixel voltage is >0. As can be seen clearly there is only one type of electric field, a vertical field, for this device. This electric field is used to turn on the device by switching the liquid crystal molecules, which is a fast process. However, when we turn off the device, the pixel voltage is either removed or reduced such that the molecules gradually relax back to a lower state. With only one type of electric field generated, this results in a very slow relaxation and hence slow turn off time, and is a major limitation of liquid crystals for many potential applications today.
Discussed below are various relevant prior art references. The references are related to three key concepts used in the present invention: crossed-field effect, fringing field switching and multi-domain technology.
The concept of crossed-field effect first appeared in 1975 in an article by D. J. Channin, Applied Physics Letters', Vol. 26, No. 11, p. 603 (1975) and in a subsequent article by D. J. Channin and D. E Carlson, Applied Physics Letters', Vol. 28, No. 6 (1976). Six years later, an article was published by Akihiko Sugimura et al., Proceedings of 14th Conference on Solid State Devices, Tokyo (1982) and in 1985 Akihiko Sugimura and Takao Kawamura published another article in, Japanese Journal of Applied Physics, Vol. 24, No. 8, p. 905 (1985). The liquid crystal displays employing the crossed-field effect have various disadvantages, such as, high voltage requirement, low contrast, more complicated structure, non-uniform transmission, and more complicated driving. Driving refers to the electronic circuits used to supply (or drive) the required voltages (data or common) to the TFT-LCD. Some driving schemes are more complicated, e.g, requiring different types of voltage at different time intervals. In the crossed field effect, it normally requires the control of two types of electric field (both vertical and lateral) using extra electrodes and hence more complicated driving. The crossed-field effect concept has therefore not been used for TFT-LCDs since it tends to require much higher operation voltage, more complicated structure and driving and have lower contrast. The present invention however improves many of the above problems by using different electrode designs; thus, making the crossed-field effect possible for use in TFT-LCDs. Moreover, the use of the crossed-field effect in the present invention also provides the inherent wide-viewing-angle property, which is another very important requirement for TFT-LCD television sets (TVs).
Prior art research on fringing-field switching (FFS) has been published by Seung Ho Hong et al., Japanese Journal of Applied Physics, “Hybrid Aligned Fringing Field” Vol. 40, p.L272, (2001) and Seung Ho Hong et al., Japanese Journal, Applied Physics, Vol 41, pp. 4571-4576 (2001). The present invention adopts a structure that is very similar to the Fringing-Field-Switching FFS mode structure described by Seung Ho Hong et al. This mode was used for generating wide-viewing-angle using in-plane-switching with improved efficiency. By adopting this structure in the present invention, the required voltage can be reduced for generating the lateral or fringing field. The reduction in voltage is possible because the gap between electrodes for fringing field generation is small. Hence, the operating voltage is lowered. Moreover, the FFS structure can provide good uniform vertical field without a dead zone, which is defined as a gap between electrodes without an electric field. In the present invention, the gap between electrodes also has an electric field generated by a bottom substrate electrode configuration that consists of an electrode layer with gaps known as a discontinuous electrode separated from a continuous electrode layer by a electrical insulation layer. The segments of the discontinuous electrode are however all connected to the same transistor within a pixel. The bottom substrate electrode structure is similar to the structure of conventional FFS structure.
The present invention however has at least three important differences from the conventional FFS structure. First, the present invention has two common electrodes; whereas, the conventional FFS structure has only one common electrode of low voltage only. A recently reported FFS mode also used two common electrodes; however, in this case both common electrodes are of low voltage, e.g. 0V. In contrast, in the present invention, one common electrode is high voltage and one common electrode is low voltage. Second, the liquid crystal (LC) mode is different. Conventional FFS uses parallel alignment with in-plane-switching whereas the recently reported FFS mode with two common electrodes uses Hybrid-Aligned-Nematic (HAN). The present invention can use any liquid crystal mode and the wide-viewing-angle generation mechanism is also different compared with the FFS Prior Arts. Third, all prior art FFS structures have slow response time since they are not using the crossed-field effect and the turn-off process relies on natural relaxation of the LC molecules and is slow.
Further, prior art references relate to multi-domain technology LCDs. The present invention adopts a mechanism of forming wide-viewing-angle known as multi-domain. The present invention however has important differences from all the prior art using this technology since our invention uses FFS structure for generating the fringing field whereas other prior art references mainly use protrusions for generating multi-domains. See A. Takeda et al., SID '98, “MVA, Multi-Domain Vertical Alignment” p. 1077 (1998). An inter-digital structure for generating the fringing field to cause multi-domain was discussed by K. H. Kim et al., SID '98 p. 1085 (1998). Moreover, the present invention can use many different liquid crystal modes compared with mainly Vertical Alignment (VA) mode used in the prior art.
Thus, there is a need for improvement in today's thin-film transistor liquid crystal display (TFT-LCD) technology. It is desirable for crossed-field effect structures to have low operation voltage, high contrast, simple driving and easy fabrication. Faster response is desired for conventional structures using FFS or multi-domain LCDs.
The present invention affords a substantial improvement in the production and performance of TFT-LCDs. Different LC modes can be applied to this structure. Different LC modes can lead to different light efficiency, response time and viewing angle. The choice of the LC mode depends on the type of application.